(1) Field of the Invention
The present invention relates to a stabilized power supply unit that is designed to supply the power from the commercial power source in the normal condition but supply the power automatically from the battery when the commercial power source has failed so that the stable power can be supplied without interruption at all times.
(2) Description of the Prior Art
FIG. 5 shows the case where the voltage of the commercial power source 1a is dropped to the specified voltage by transformer 1 installed on a utility pole, added to full-wave rectification circuit 4 through AC input terminals 2 and 3 for being rectified by said full-wave rectification circuit 4, smoothed by capacitor 5 and inputted to DC-AC inverter 6. The DC-AC inverter 6 serves to convert the DC voltage into AC voltage and obtain the predetermined AC voltage between the AC output terminals 8 and 9 through the isolation transformer 7 for the supply to the load.
When the commercial power source has failed due to an accident and the like, this is detected by a detector (not shown) to cause switch 10 to close, and battery 11 supplies the power to the DC-AC inverter 6 so that the AC output can be obtained without any interruption.
When the DC-AC inverter has got out of order to cause the rapid variation of the output voltage or to cause an excessive load current to flow through the load connected to both the AC output terminals 8 and 9, this is detected by output voltage detection circuit 12 or load current detection circuit 13, and change-over switch 14 is switched directly to line 15 on the AC input terminal 2 to prevent the supply of the power from being interrupted.
As described in the foregoing, in the case of what is called the direct-supply circuit designed for the direct supply of the AC input voltage through the AC input terminal 2 connected directly to the load, it is necessary for the side of the AC input terminal 3 to be connected to the AC output terminal 9 through common line 16. If these AC input terminals 8 and 9 are not connected to each other through the common line 16, the potential difference will occur between the terminals 8 and 9 when, for example, an impedance element is inserted in between, and this makes the grounding unstable.
In connecting the AC input terminals 8 and 9 to each other through a common line 16 in the manner common to the conventional circuit, however, there has been the problem such that the isolation device such as the transformer 7 has to be interposed between the AC input sides 2 and 3 and the output side of DC-AC inverter 6 in consideration of the relation with the internal potential. Thus, an object of the present invention is to provide a power supply unit capable of composing a direct-supply circuit not requiring the isolation means such as the transformer. Another object of the present invention is to provide a power supply unit featuring the reduced number of parts, compactness, lightweight, lower price and higher reliability. The further objects of and the features of the present invention will be clarified through the explanations in the following.